// Lissajous generates GIF animations of random Lissajous figures.
package main

import (
	"fmt"
	"image"
	"image/color"
	"image/gif"
	"io"
	"math"
	"math/rand"
	"os"
	"time"
)

var palette = []color.Color{color.White, color.RGBA{0x00, 0xff, 0x00, 0xff}, color.RGBA{0xff, 0x00, 0x00, 0xff}, color.RGBA{0x00, 0x00, 0xff, 0xff}}

const (
	whiteIndex = 0 // first color in palette
	blackIndex = 1 // next color in palette
)

func main() {
	// The sequence of images is deterministic unless we seed
	// the pseudo-random number generator using the current time.
	// Thanks to Randall McPherson for pointing out the omission.
	rand.Seed(time.Now().UTC().UnixNano())
	// 这里修改一下，让输出写入到文件
	// lissajous(os.Stdout)

	f, err := os.Create(os.Args[1])
	if err != nil {
		fmt.Fprintf(os.Stderr, "lissajous: %v\n", err)
	}
	lissajous(f)
	f.Close()

}

func lissajous(out io.Writer) {
	// 常量的定义，定义之后不能更改
	// 这里是一种批量定义方式
	// 变量的类型编译器会根据后面的参数进行猜测
	const (
		cycles  = 5     // number of complete x oscillator revolutions
		res     = 0.001 // angular resolution
		size    = 100   // image canvas covers [-size..+size]
		nframes = 64    // number of animation frames
		delay   = 8     // delay between frames in 10ms units
	)

	freq := rand.Float64() * 3.0 // relative frequency of y oscillator
	anim := gif.GIF{LoopCount: nframes}
	phase := 0.0           // phase difference
	colorIndex := uint8(0) // 颜色索引
	for i := 0; i < nframes; i++ {
		rect := image.Rect(0, 0, 2*size+1, 2*size+1)
		img := image.NewPaletted(rect, palette)
		for t := 0.0; t < cycles*2*math.Pi; t += res {
			x := math.Sin(t)
			y := math.Sin(t*freq + phase)

			// 调色板选择其它颜色，让它变成彩色的
			colorIndex = uint8(t)%3 + 1
			img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5),
				colorIndex)
		}
		phase += 0.1
		anim.Delay = append(anim.Delay, delay)
		anim.Image = append(anim.Image, img)
	}
	gif.EncodeAll(out, &anim) // NOTE: ignoring encoding errors
}
